The presently claimed invention relates generally to a ceramic composite and related articles for selective emission of radiation.
There are many classes and types of lighting devices available on the market today including incandescent lamps, discharge based lamps such as high intensity discharge (HID) and fluorescent lamps, as well as solid state devices such as Light Emitting Diodes (LEDs) and Organic LEDs (OLEDS). Each of these devices has certain advantages and disadvantages depending upon the application within which they are to be used.
Tungsten filament incandescent lamps for example have numerous advantages for indoor and outdoor lighting systems. These advantages include simplicity of use, pleasing color, instant start, dimmability and low cost, not to mention a very large installation base. However, because much of the input energy of incandescent lamps is radiated outside the visible spectrum, incandescent lamps tend to have low energy efficiencies (e.g., on the order of 17 lumens per watt (LPW) for a 100 watt (100 W) lamp rated at 120 volts (120V) and having a rated lifetime of 750 hours). In particular, only about nine percent (9%) of power supplied to incandescent lamps is radiated as visible light with the remaining power being radiated as waste heat. Despite the many inherent advantages of incandescent lamps, if their efficiency cannot be improved, they will continue to lose market share to compact fluorescent lamps, which have an advantage in efficacy, albeit at the expense of color, dimmability, and acquisition cost.
It has been suggested that one possible approach to improve the efficiency of incandescent lamps is through the use of photonic crystals to modify or suppress thermal radiation above a cutoff wavelength. However, all such suggested photonic crystal designs are limited by one or more factors including the materials and lattice structures employed, as well as the resulting efficiencies afforded.
For example, in U.S. Pat. No. 6,768,256 issued to Sandia Corporation (hereinafter the '256 patent), a photonic crystal light source is described that is said to provide an enhanced light emission at visible and infrared wavelengths (e.g., enhanced photonic density-of-states). In the '256 patent, the photonic crystal structure is configured in an inherently unstable stacked log pile design utilizing alternating layers of tungsten rods in an attempt to create a photonic band gap. Although some enhanced light emission is reported, the spacing between the tungsten rods ranges from 2.8 μm with a rod width of 1.2 μm to 4.2 μm with a rod width of 0.85 μm. This results in a band edge for the allowed band of energies occurring beyond 4 μm yielding a minimal increase in efficiency. In order for such a tungsten log pile design to provide a band gap that is applicable in a lighting device such as an incandescent lamp, the lattice spacing would need to be on the order of about 400 nm. However, at such a small scale, 400 nm tungsten rods become extremely unstable when exposed to temperatures common to an incandescent environment (e.g., at or above 1700 Kelvin) for as little as two hours.
FIGS. 1(A-C) illustrate an example of 400 nm tungsten rods having been exposed to temperatures of 300 Kelvin, 1500 Kelvin and 1700 Kelvin, respectively for a period of two hours. With reference to FIGS. 1(A-C) it can be easily seen that as the temperature is increased, the grain size within the rods increases toward the feature size causing the rods to become unstable. Similarly, other mechanisms such as Raleigh instability may cause the logs to spheroidize into droplets rendering the structures unstable at high temperatures.
Thus, although the prior art may suggest methods of improving efficiencies of incandescent lamps, all such suggested improvements fail to teach material and structural combinations at the appropriate scale that are predicted to be stable at temperatures above 1700 Kelvin for extended periods of time.